Hydrophilizing agent for hydrophobic porous membrane, and method for hydrophilizing hydrophobic porous membrane and test method using this agent

ABSTRACT

A hydrophilizing agent for a hydrophobic porous membrane, wherein the agent contains a surfactant, and the surfactant has a frothability such that the bubble height immediately after frothing, as measured according to the Ross-Miles method (JIS K 3362), using a 0.1 wt % aqueous solution of the surfactant at 25° C., is 40 mm or less, and preferably has a frothability such that the bubble height five minutes after frothing is 20 mm or less; a method for hydrophilizing a hydrophobic porous membrane using this hydrophilizing agent; and a method for testing and hydrophilizing a membrane module using this hydrophilizing agent.

TECHNICAL FIELD

This invention relates to a hydrophilizing agent that is used for ahydrophobic porous membrane and contains a low-frothing surfactant, andmore particularly relates to the use of the above-mentioned hydrophobicporous membrane in the hydrophilization treatment of a hydrophobicporous membrane, such as a microfiltration membrane or anultrafiltration membrane.

BACKGROUND ART

Porous membranes, such as microfiltration membranes or ultrafiltrationmembranes, have been used in a wide range of fields for such purposes astreating industrial waste and other such sludge substances, andsterilizing pharmaceutical water. Membranes used for these purposes canbe broadly divided into hydrophobic porous membranes and hydrophilicporous membranes, and in the field of solid-liquid separation,hydrophobic porous membranes are preferable because of their chemicalresistance, fouling resistance, weather resistance, resistance todegradation by oxidation, and so forth. However, water or an aqueoussolution cannot pass through the pores of a hydrophobic porous membranebecause of the hydrophobic nature of the membrane, or considerablepressure must be applied even if such passage is possible. Therefore,hydrophobic porous membranes are subjected to a hydrophilizationtreatment ahead of time so that water or an aqueous solution can passthrough more easily. This hydrophilization treatment is necessary notonly when a hydrophobic porous membrane is to be used for the first timeafter its manufacture, but also whenever all or part of the membranedries out through contact with air during the inspection or cleaning ofthe membrane or during an extended shut-down of operation. Theappropriate hydrophilization of a hydrophobic porous membrane isparticularly important with a membrane composed of a polymer that ishighly hydrophobic, such as a fluororesin, because there is a pronounceddecrease in liquid permeability once the membrane is dried.

Examples of known methods for hydrophilizing a hydrophobic porousmembrane include a method in which hydrophilic groups are introducedinto the membrane itself (Japanese Laid-Open Patent ApplicationH6-296686), as well as treating the membrane with deaerated water(Japanese Laid-Open Patent Application H5-208121), alcohol (JapaneseLaid-Open Patent Application S58-96633), glycerol (Japanese Laid-OpenPatent Application 2002-95939), or an inorganic salt (Japanese Laid-OpenPatent Application H6-277470).

However, with a method in which hydrophilic groups are introduced intothe membrane itself (Japanese Laid-Open Patent Application H6-296686),the membrane has to be thoroughly washed with a cleaning liquid, such asa large amount of water, to remove any of the monomer constituting thehydrophilic groups that remains unreacted on the membrane. With a methodin which the membrane is treated with deaerated water (JapaneseLaid-Open Patent Application H5-208121), deaerated water essentially hasto be pressurized and passed through the membrane, so this treatmentmethod is complicated. Furthermore, since the hydrophilized membranemust be kept moist at all times, a module containing the hydrophilizedmembrane has to be transported, shipping, sold, and so forth in a stateof being filled with moisturizing liquid or the like, which means thatit handling is inconvenient. With a method in which the membrane istreated with alcohol or the like (Japanese Laid-Open Patent ApplicationsS58-96633, etc.), the alcohol or other substance used for the treatmentremains in the hydrophobic porous membrane, so the membrane has to bethoroughly washed with a large amount of cleaning liquid at the time ofits use.

Also, a method in which a specific process is employed to treat amembrane with a surfactant has been disclosed as anotherhydrophilization treatment method for a hydrophobic porous membrane(Japanese Laid-Open Patent Application H1-119310).

Japanese Laid-Open Patent Application H1-119310, however, is employedthe specific process to reduce the amount of surfactant used and therebyminimize a drawback of elution on the basis that there is the drawbackin that when a membrane is treated with a surfactant, some of thesurfactant remains behind and is gradually eluted into the treatedwater. In other words, Japanese Laid-Open Patent Application H1-119310merely involves reducing the amount of surfactant used and therebyreducing the amount of surfactant that is eluted, and does not solve thefundamental problem of surfactant elution.

DISCLOSURE OF THE INVENTION

It is a first object of the present invention to provide ahydrophilizing agent that is suited to hydrophilizing a hydrophobicporous membrane, and a hydrophilization method in which thishydrophilizing agent is used.

It is a second object of the present invention to provide ahydrophilizing agent for hydrophilizing a hydrophobic porous membrane,with which the amount of hydrophilizing agent remaining after thetreatment of a hydrophobic porous membrane is dramatically reduced, anda hydrophilization method in which this hydrophilizing agent is used.

It is a third object of the present invention to provide a method fortesting a membrane module, with which bubbles generated in ahydrophilizing agent during testing for leaks, defects, clogging, or thelike of the membrane module can be favorably suppressed, with themembrane module containing a hydrophobic porous membrane and beingimmersed in the hydrophilizing agent.

It is a fourth object of the present invention to provide a favorablemethod for hydrophilizing a membrane module containing a hydrophobicporous membrane and disposed in a membrane separation tank containing asolution to be treated (treatment liquid) and equipped with an aerationapparatus, while the membrane module is still immersed in the treatmentliquid.

As a result of diligent study aimed at solving the above problems, theinventors arrived at the present invention upon discovering that theseproblems could be solved by using a specific low-frothing surfactanthaving a defoaming property and low surface tension.

Specifically, the present invention relates to:

1. A hydrophilizing agent for a hydrophobic porous membrane, wherein theagent contains a surfactant, and the surfactant has a frothability suchthat the bubble height immediately after frothing, as measured accordingto the Ross-Miles method (JIS K 3362), using a 0.1 wt % aqueous solutionof the surfactant at 25° C., is 40 mm or less.

2. The hydrophilizing agent for a hydrophobic porous membrane accordingto 1 above, wherein the surfactant has a frothability such that thebubble height five minutes after frothing, as measured according to theRoss-Miles method (JIS K 3362), using a 0.1 wt % aqueous solution of thesurfactant at 25° C., is 20 mm or less.

3. The hydrophilizing agent for a hydrophobic porous membrane accordingto 1 or 2 above, wherein the surfactant has a static surface tension of30 mN/m or less when a 0.1 wt % aqueous solution of the surfactant isused.

4. The hydrophilizing agent for a hydrophobic porous membrane accordingto any of 1 to 3 above, wherein the surfactant is acetylene glycol,ethoxylated acetylene glycol, or a mixture of these.

5. A method for hydrophilizing a hydrophobic porous membrane, comprisinga step of bringing a hydrophobic porous membrane into contact with thehydrophilizing agent for a hydrophobic porous membrane according to anyof 1 to 4 above.

6. The method for hydrophilizing a hydrophobic porous membrane accordingto 5 above, further comprising a step of drying the hydrophobic porousmembrane that has come into contact with the hydrophilizing agent for ahydrophobic porous membrane according to any of 1 to 4 above.

7. A method for testing a membrane module having a main body, an inletand outlet provided to the main body, and a hydrophobic porous membraneprovided inside the main body, the method comprising the steps of:

(1) immersing the membrane module in the hydrophilizing agent for ahydrophobic porous membrane according to any of 1 to 4 above;

(2) introducing a test gas through the inlet, allowing the gas to passthrough the hydrophobic porous membrane, and discharging the gas throughthe outlet; and

(3) observing bubbles discharged from the membrane module.

8. A method for testing and hydrophilizing a membrane module having amain body, an inlet and outlet provided to the main body, and ahydrophobic porous membrane provided inside the main body, the methodcomprising the steps of:

(1) immersing the membrane module in the hydrophilizing agent for ahydrophobic porous membrane according to any of 1 to 4 above;

(2) introducing a test gas through the inlet, allowing the gas to passthrough the hydrophobic porous membrane, and discharging the gas throughthe outlet;

(3) observing bubbles discharged from the membrane module; and

(4) drying the membrane module.

By immersing a membrane that is highly hydrophobic, such as afluorine-based separation membrane, for a specific time in thelow-frothing surfactant of the present invention and then drying themembrane, this separation membrane can be stored stably in a dry statewithout degrading for an extended period, and when this separationmembrane is to be used, it can be spontaneously and completely moistenedwith water, which means that dry storage is facilitated, and anexcellent hydrophobic porous membrane that needs no pretreatment at thetime of its use can be provided.

Also, with the method of the present invention for hydrophilizing ahydrophobic porous membrane, a porous membrane that has been dried andbecome hydrophobic can be restored to good liquid permeability ineasily, smoothly, and inexpensively, in a short time, with little labor,and using a small amount of chemicals, all of which is extremelyadvantageous for industrial purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a preferred example for working themethod of the present invention for hydrophilizing a membrane.

BEST MODE FOR CARRYING OUT THE INVENTION (1) Hydrophilizing Agent for aHydrophobic Porous Membrane

The hydrophilizing agent for a hydrophobic porous membrane of thepresent invention contains a low-frothing surfactant and any solvent andany additives desired.

(1-1) Surfactant

The surfactant of the present invention has low frothability. Thefrothability can be measured according to the Ross-Miles method (JIS K3362). For instance, a 0.1 wt % aqueous solution of surfactant wasfrothed at 25° C. using an apparatus conforming to the froth strengthmeasurement apparatus set forth in JIS K 3362, and the bubble heightimmediately after frothing and the bubble height five minutes afterfrothing were measured according to the Ross-Miles method (JIS K 3362).The Ross-Miles method mentioned here involves putting 50 mL of asurfactant aqueous solution in a glass cylinder with an inside diameterof 50 mm, dropping 200 mL of surfactant aqueous solution on top of thisfrom a height of 90 cm for 30 seconds, and measuring the bubble height(mm) immediately after dropping and after a specific time has elapsed.It is favorable for the surfactant of the present invention to be suchthat the bubble height immediately after frothing, as measured by theabove-mentioned Ross-Miles method, is 40 mm or less, and preferably 30mm or less, and even more preferably 20 mm or less. 40 mm or less isfavorable because foaming by the surfactant can be kept low.

Also, it is favorable for the surfactant of the present invention to besuch that the bubble height five minutes after frothing, as measured bythe above-mentioned Ross-Miles method (JIS K 3362), is 20 mm or less,and preferably 15 mm or less, and even more preferably 0 to 10 mm. 20 mmor less is favorable because foaming by the surfactant can be kept low.

The surfactant used in the present invention preferably has a staticsurface tension of 29 mN/m or less when a 0.1 wt % aqueous solution ofthe surfactant is used. Even more preferable is 28 mN/m or less, with arange of 20 to 28 mN/m being better yet. The static surface tension herecan be measured with a CBVP-Z Automatic Surface Tensiometer (made byKyowa Interface Science) that features the JIS Wilhelmy (plate) method.It is preferable for the static surface tension to be 30 mN/m or lessbecause the hydrophilization of the hydrophobic porous membrane willtend to take less time.

Also, the surfactant used in the present invention preferably has adynamic surface tension (room temperature) of 50 mN/m or less when a 0.1wt % aqueous solution of the surfactant is used. Even more preferable isa range of 10 to 50 mN/m, with 25 to 40 mN/m being better yet. Thedynamic surface tension can be measured from the values obtained at 1 Hzand 10 Hz for a 0.1 wt % aqueous solution, using a Bubble PressureDynamic Bubble Pressure Tensiometer, Krüss BP-2 (made by Krüss), forexample.

Surfactants that can be used in the present invention can be selectedfrom among anionic surfactants, cationic surfactants, amphotericsurfactants, and nonionic surfactants. A nonionic surfactant isparticularly favorable from the standpoints of foaming and frothing.

Specific examples of nonionic surfactants include acetylene glycol-basedsurfactants, acetylene alcohol-based surfactants, polyoxyethylene nonylphenyl ether, polyoxyethylene octyl phenyl ether, polyoxyethylenedodecyl phenyl ether, polyoxyethylene alkyl allyl ether, polyoxyethyleneoleyl ether, polyoxyethylene lauryl ether, polyoxyethylene alkyl ether,polyoxyalkylene alkyl ether, and other such ether-based surfactants,polyoxyethylene oleic acid, polyoxyethylene oleic acid esters,polyoxyethylene distearic acid esters, sorbitan laurate, sorbitanmonostearate, sorbitan monooleate, sorbitan sesquioleate,polyoxyethylene monooleate, polyoxyethylene stearate, and other suchester-based surfactants, dimethylpolysiloxane and other suchsilicon-based surfactants, and fluoroalkyl esters, perfluoroalkylcarboxylates, and other such fluorine-containing surfactants.

Of the above-mentioned nonionic surfactants, acetylene glycol-basedsurfactants are particularly favorable because of their excellentwettability, permeability, and defoaming property. In addition, anacetylene glycol surfactant is a relatively stable substance, and willnot undergo biodegradation even when a membrane is stored for anextended period. Another characteristic of an acetylene glycolsurfactant is its high permeability, and particularly its low dynamicsurface tension. Accordingly, it can be used favorably in thehydrophilization treatment of a relatively thick hollow fiber membrane,and treatment can be completed in a short time, among other benefits.

Specific examples of acetylene glycol surfactants include2,4,7,9-tetramethyl-5-decyn-4,7-diol, 3,6-dimethyl-4-octyn-3,6-diol,3,5-dimethyl-1-hexyn-3-ol, 2,5,8,11-tetramethyl-6-dodecyn-5,8-diol, andethoxylates of these.

One or more of these can be suitably selected as dictated by thesituation, but of these, it is preferable to use one of theabove-mentioned ethoxylates in which the total number of added moles ofethylene oxide is between 2 and 30. A range of 4 to 12 moles is evenbetter. Keeping the total number of added moles of ethylene oxide to 30or less lowers the static and dynamic surface tension, and the productcan be used favorably as a hydrophilizing agent.

Acetylene glycol surfactants and ethoxylates thereof can also bepurchased as commercially available products, examples of which includeSurfynol 104, 82, 465, 485, and TG made by Air Products, and Olfine STG,Olfine E1010, Olfine EXP4036, and Olfine PD-001 made by Nissin ChemicalIndustry.

For example, Olfine EXP4036 (made by Nissin Chemical Industry), which isa type of acetylene glycol surfactant, has a static surface tension of30 mN/m or less at 0.1 wt %. Olfine PD-001 and Olfine STG (both made byNissin Chemical Industry) similarly have a static surface tension of 30mN/m or less at 0.1 wt %. Thus, an acetylene glycol surfactant canexhibit good hydrophilicity at an extremely low concentration.

(1-2) Solvent

Water, physiological saline or another aqueous solution containing anelectrolyte, ethanol, methanol, or another C₁ to C₄, and preferably C₁or C₂, lower alcohol, pyridine, chloroform, cyclohexane, ethyl acetate,toluene, or a mixture of these can be used as a solvent for dissolvingthe surfactant of the present invention. The use of water isparticularly favorable from the standpoints of the effect on thematerials used to perform the hydrophilization treatment, post-treatmentof the solvent, safety, cost, and so forth. It is especially good forthe water that is used to be ordinary tap water or deionized water thathas been filtered through a hollow fiber membrane with a pore size of0.01 to 1 μm.

(1-3) Additives

Additives can optionally be added to the hydrophilizing agent for ahydrophobic porous membrane of the present invention. Examples ofadditives that can be used include surfactants other than theabove-mentioned one, and glycerol.

For example, ethylene oxide or propylene oxide, or a mixture of these,or a block polymer thereof (such as Epan 750 made by Dai-ichi KogyoSeiyaku), can be used for the purpose of increasing the water solubilityof the surfactant used in the present invention.

These additives can be used to the extent that they do not compromisethe characteristics of the hydrophilizing agent of the presentinvention. For instance, they can be used in an amount of 5 to 90 wt %,and preferably 5 to 50 wt %, with respect to the entire hydrophilizingagent. Furthermore, pure water or a water-soluble organic solvent can beused to the extent that it does not compromise the characteristics ofthe hydrophilizing agent of the present invention, and can be used, forexample, in an amount of 25 wt % or less, and preferably 10 to 20 wt %,with respect to the entire hydrophilizing agent.

(1-4) Preparation of a Hydrophobic Porous Membrane Hydrophilizing Agent

The hydrophobic porous membrane hydrophilizing agent of the presentinvention is prepared by dissolving the above-mentioned surfactant in asolvent, either by itself or along with any optional additives. Anexample of how the surfactant is dissolved is to mix the components by aknown mixing preparation method, such as using a propeller type stirrer.Components that are solids at normal temperature can be heated asnecessary before mixing.

The hydrophobic porous membrane hydrophilizing agent of the presentinvention preferably contains the above-mentioned surfactant in anamount of 0.05 to 5 wt %, and even more preferably 0.05 to 1 wt %, withrespect to the entire hydrophobic porous membrane hydrophilizing agent.Keeping the surfactant content to 0.05 wt % or higher tends tocontribute to excellent characteristics as a hydrophilizing agent, whilekeeping the surfactant content to 5 wt % or less tends to reduce theamount of elution from the membrane and lower the COD.

(2) Membrane Module

The hydrophobic porous membrane hydrophilizing agent of the presentinvention is used to hydrophilize a hydrophobic porous membrane in amembrane module. Any of various types of membrane module can be used,such as a flat membrane type, cylindrical type, pleated type, or hollowfiber type.

(2-1) Structure of Membrane Module

The membrane module has a main body, an inlet, an outlet, and a porousmembrane. More specifically, the inlet and outlet are provided to themembrane module main body, and the porous membrane is provided insidethe main body. The inlet and outlet may be provided to the two ends ofthe main body (a type that is linear and open at both ends), or eitherthe inlet or the outlet may be opened large (a type that is linear andopen on one side). The porous membrane is linked to the inside of themain body so as to divide the main body into to a first chamber havingan inlet and a second chamber having an outlet. This linking includesadhesive bonding or sealing of the ends of the porous membrane to theinner walls of the main body, or the ends of the porous membrane may bedetachably connected to the main body inner walls. Therefore, themembrane module of the present invention has a structure with which aliquid or gas that has been introduced through the inlet enters the mainbody, constantly passes through the porous membrane, and is dischargedthrough the outlet.

The inlet, outlet, and main body of the membrane module of the presentinvention may be made of stainless steel, [ordinary] steel, or anothersuch metal, or of a fluororesin, ABS resin, polyolefin resin, vinylchloride resin, or other such resin.

(2-2) Hydrophobic Porous Membrane

The hydrophobic porous membrane of the present invention can be anyporous membrane as long as it is a porous membrane that is hydrophobic.Examples of the shape of the hydrophobic porous membrane of the presentinvention include a flat membrane, a hollow fiber membrane, a tubularmembrane, and a spiral membrane. The hydrophobic porous membrane of thepresent invention may also be a microfiltration membrane (MF), anultrafiltration membrane (UF), a nanofiltration membrane (NF), oranother such separation membrane.

The hydrophobic porous membrane of the present invention can be formedfrom any of various materials, as long as the material can be formed inthe shape of a separation membrane, such as a material based oncellulose, polyolefin, polyvinyl alcohol, polysulfone,polyacrylonitrile, or a fluororesin. Examples include polyethylene,polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, andpolysulfone. The use of a highly hydrophobic resin is particularlyfavorable in terms of the surface characteristics of the hydrophobicporous membrane, and a fluororesin is especially good. Amongfluororesins, it is even better to use a vinylidene fluoride resinbecause of its good chemical resistance and because it can be easilyshaped into a film. Examples of vinylidene fluoride resins here includea homopolymer of vinylidene fluoride, as well as a copolymer ofvinylidene fluoride and a monomer that can be copolymerized withvinylidene fluoride. Examples of the above-mentioned copolymerizablemonomers include vinyl fluoride, tetrafluoroethylene, trifluoroethylene,and hexafluoropropylene.

The hydrophobic porous membrane of the present invention has a pluralityof pores. These pores are preferably continuous pores that go all theway through the front and back sides of the hydrophobic porous membrane.The pore diameter can be selected as dictated by the intendedapplication, but a suitable range, for example, is 0.01 to 5 μm, andpreferably 0.1 to 1 μm. It is also preferable for the hydrophobic porousmembrane of the present invention to have an asymmetric structure inwhich the pore diameter is small on one side of the hydrophobic porousmembrane and large on the other side. In the case of an asymmetricstructure, it is favorable for the pore diameter on one side to be morethan 1 time and not more than 100 times, and preferably from 2 to 10times, the pore diameter on the other side.

If the hydrophobic porous membrane is a hollow fiber membrane, theoutside diameter of the hollow fiber is, for example, 0.1 to 10 mm, andpreferably 0.5 to 5 mm. It is favorable for the hydrophobic porousmembrane of the present invention to have a pure water permeationcoefficient, which indicates the liquid permeation performance withrespect to pure water, of 10 to 250 m³/m²/hr/MPa, and preferably 20 to150 m³/m²/hr/MPa. The pure water permeation coefficient can becalculated from the following equation.

Pure water permeation coefficient={amount of pure water permeation(m³)}/{surface area of porous membrane (m²)}/{permeation time(hr)}/{pressure of pure water (MPa)}

(3) Method for Hydrophilizing Hydrophobic Porous Membrane

There are normally cases when the surface of a hydrophobic porousmembrane comes into contact with the air and is in a dry state, such aswhen the porous membrane is used for the first time, when the membraneis replaced, when the membrane is washed with a chemical, or when themembrane is not used for an extended period. In such cases, if themembrane is immersed again in the liquid to be treated (treatmentliquid) or the like and an attempt is made to filter the treatmentliquid, liquid permeation performance will suffer and the membrane willbe unable to perform its original function as a separation membrane.Therefore, a membrane with both good fouling resistance and enhancedliquid permeability can be obtained by passing the treatment liquidthrough the membrane after first hydrophilizing the pores of thehydrophobic porous membrane. In terms of eliminating the inefficiency ofrecovering treated liquid as waste liquid, being able to use thehydrophobic porous membrane sooner, and so forth, it is preferable forthe hydrophilizing agent to be quickly removed by passing a liquidthrough the hydrophobic porous membrane.

The hydrophilization of the hydrophobic porous membrane here includesbringing the hydrophobic porous membrane into contact with theabove-mentioned hydrophobic porous membrane hydrophilizing agent. Anexample of hydrophilizing a hydrophobic porous membrane in a membranemodule will now be described.

A hydrophilization treatment using the hydrophobic porous membrane ofthe present invention in a membrane module is conducted by injecting theabove-mentioned hydrophobic porous membrane hydrophilizing agent fromthe side of the hydrophobic porous membrane facing the second chamberhaving the outlet. If there are two or more outlets, then (a) thehydrophobic porous membrane hydrophilizing agent may be injected throughall of the outlets, or (b) the hydrophobic porous membranehydrophilizing agent may be injected through one or more of the outlets,and any extra hydrophobic porous membrane hydrophilizing agent may bedischarged through the remaining outlets.

(a) When the hydrophobic porous membrane hydrophilizing agent isinjected through all of the outlets, any gas that has collected in theoutlets or the second chamber provided with the outlets is pushed by thehydrophobic porous membrane hydrophilizing agent into the first chamberprovided with an inlet. As a result, the hydrophobic porous membrane isgradually hydrophilized from the second chamber side toward the firstchamber side.

(b) When the hydrophobic porous membrane hydrophilizing agent isinjected through one or more of the outlets, and any extra hydrophobicporous membrane hydrophilizing agent is discharged through the remainingoutlets, any gas that has collected in the outlets or the second chamberprovided with the outlets can be pushed from one outlet to anotheroutlet by the hydrophobic porous membrane hydrophilizing agent.

A hollow fiber membrane module is particularly prone to the collectionof bubbles because of its complex structure.

If all of the inlets and outlets of a membrane module installed in atreatment water tank have a structure that is less likely to createareas of collection of air or the like above the water surface in theabove-mentioned water tank, then any bubbles inside the membrane modulecan be allowed to escape, and the entire membrane module filled withhydrophilizing agent, by using a pump or the like to push thehydrophilizing agent in from the outlet at a specific pressure and flux.

However, if at least one part of the water collection component of themembrane module installed in the treatment water tank is below themembrane module, then it is preferable to employ a method in which apump or the like is used to push the hydrophilizing agent in from thewater collection component below the membrane module, push any gasremaining inside the membrane module and inside the piping out from thewater collection component located above the membrane module, replacethe bubbles with hydrophilizing agent, and thereby fill the entiremembrane module with hydrophilizing agent.

A good hydrophilization treatment can be carried out with either (a) or(b) above, but from the standpoint of creating a structure thatfacilitates routing of the piping provided to the membrane module outletwhen hydrophilization is performed, it is preferable to use method (a),in which the hydrophobic porous membrane hydrophilizing agent isinjected through all of the outlets.

When the concentration of surfactant is 0.3 wt %, the flow amount of thehydrophobic porous membrane hydrophilizing agent of the presentinvention here is, for example, 0.5 to 5 liters, and preferably 2 to 3liters, per square meter of membrane surface area. If the flow amount isat least 0.5 liter per square meter of membrane surface area, thehydrophilization of the membrane can be expected to have an adequateeffect. Also, if the flow amount is no more than 5 liters per squaremeter of membrane surface area, the membrane separation will not besubjected to excessive load.

The injection rate of the hydrophobic porous membrane hydrophilizingagent is, for example, from 0.005 to 3 m³/m²·D, and preferably 0.01 to0.3 m³/m²·D, per unit of membrane surface area. If the injection rate isno more than 3 m³/m²·D, the entire membrane surface can be uniformlyhydrophilized, and if it is at least 0.005 m³/m²·D, the hydrophilizationtreatment can be carried out more quickly.

The adhesion proportion of hydrophobic porous membrane hydrophilizingagent that adheres to the hydrophobic porous membrane is, for example,0.01 to 1.0 wt %, and preferably 0.05 to 0.5 wt %. This adhesionproportion can be found by measuring the weight (W₀) (g) of thehydrophobic porous membrane prior to the hydrophilization treatment, andthe weight (W₁) (g) of the hydrophobic porous membrane after it hasundergone hydrophilization treatment and been dried, and thencalculating from the following equation.

Adhesion proportion (%)={(W ₁ (g)−W ₀ (g))/W ₀ (g)}×100

The hydrophilicity will be good if the adhesion proportion is at least0.01 wt %, but the adhesion proportion is preferably no more than 1.0 wt% so that no extra hydrophobic porous membrane hydrophilizing agent willbe included in the membrane module.

The temperature of the hydrophobic porous membrane hydrophilizing agentof the present invention during hydrophilization is, for example, from10 to 50° C., and preferably 20 to 30° C. If the temperature is 10° C.or higher, there will be no decrease in the hydrophilization rate andpermeation performance will be enhanced because the hydrophilizationtreatment is performed more thoroughly. If the temperature is 50° C. orlower, there will be no decrease in permeation performance due tothermal degradation of the hydrophilizing agent or to heat shrinkage ofthe membrane. The hydrophilizing agent may be recovered immediatelyafter the injection of the hydrophobic porous membrane hydrophilizingagent, but the immersion time in the hydrophobic porous membranehydrophilizing agent is at least 30 seconds, and allowing thehydrophobic porous membrane to soak in the hydrophobic porous membranehydrophilizing agent for 10 to 120 minutes, and preferably 30 to 90minutes, is even better because the hydrophilization will be morecomplete and permeation performance will be enhanced.

The hydrophobic porous membrane hydrophilizing agent of the presentinvention that is contained in the membrane module can be suitablyrecovered after the hydrophilization treatment by tilting the membranemodule, for example, and thereby discharging any extra hydrophilizingagent contained in the module. The hydrophobic porous membranehydrophilizing agent can be pushed out by allowing water or the liquidto be treated to flow from the first chamber side having the inlet tothe second chamber side having the outlet. This further improves theefficiency of hydrophilizing the membrane, and also cuts down on thework entailed by recovery. The water used for this purpose is preferablypure water, refined water, or other such water that is clean enough notto foul the second chamber side having the outlet. It is even morefavorable to use bactericidal liquid such as a sodium hypochloriteaqueous solution. This may also be water obtained by filtering ordinarytap water or deionized water through a hollow fiber membrane with a poresize of 0.01 to 1 μm.

The method of the present invention for hydrophilizing a hydrophobicporous membrane will be described in further detail through reference toFIG. 1.

FIG. 1 is a simplified diagram of a membrane separation apparatusincluding the membrane module of the present invention. The example hereis a membrane separation apparatus for the treatment of a liquid to betreated (treatment liquid), which contains organic matter, for example,with microbes and a separation membrane. The treatment liquid 3 is firstintroduced into a membrane separation tank 1, and treated with microbesin the membrane separation tank 1. The organic matter here usuallyincludes proteins, amino acids, sugars, lipids, or other biodegradablesubstances, and this organic matter can be significantly removed withmicrobes and the membrane module of the present invention. Thesemicrobes can be those contained in active sludge or the like, or thoseused in a bioreactor for producing useful substances. In this microbialtreatment, air is blown from a diffuser pipe 4 into the membraneseparation tank 1. This liquid treated with the microbes is treated asit passes through a hydrophobic porous membrane (not shown) in amembrane module 2, and this treated liquid is discharged through a pipe5.

When the hydrophobic porous membrane of the present invention ishydrophilized, first a hydrophobic porous membrane hydrophilizing agentis introduced from the pipe 5 side into the hydrophobic porous membrane(not shown) in the membrane module 2. If there are a plurality ofoutlets in the membrane module 2, excess hydrophobic porous membranehydrophilizing agent may be discharged from one outlet into the membraneseparation tank 1. After this, the hydrophobic porous membranehydrophilizing agent is held for a specific length of time in thehydrophobic porous membrane. After this time has elapsed, fresh water isintroduced from the pipe 5 side into the hydrophobic porous membrane(not shown) in the membrane module 2, to replace the hydrophobic porousmembrane hydrophilizing agent with water. In this replacement, air maybe blown from the diffuser pipe 4 into the membrane separation tank 1.As a result, hydrophilization is carried out, and the concentration ofhydrophobic porous membrane hydrophilizing agent contained in theinitial flow of treatment liquid after the hydrophilization treatmentcan be suitably lowered.

Thus, with the hydrophilization method of the present invention, inwhich a hydrophobic porous membrane hydrophilizing agent that is anaqueous solution of a surfactant with low frothing is injected from atreatment liquid side, even if at least a part of the hydrophobic porousmembrane should dry out have diminished permeation performance (membraneflux), restoring the membrane flux will entail less hydrophilizingagent, labor, time, and expense. Also, since a surfactant with lowfrothing is used, even if some of the surfactant should flow into themembrane separation tank 1 as a result of the hydrophilizationtreatment, and gas is subsequently bubbled from the diffuser pipe 4,foaming by the surfactant can be kept to a minimum, and no bubbles ortreatment liquid 3 will leak out from the membrane separation tank 1.Furthermore, since the hydrophobic porous membrane hydrophilizing agentcan be replaced by flushing with a small amount of water after thehydrophilization treatment, admixture of the hydrophobic porous membranehydrophilizing agent into the treatment liquid can be suppressed, so themembrane separation apparatus can be put back in operation moresmoothly. In addition, with a conventional hydrophilization method inwhich ethanol or the like is used, the ethanol may flow into themembrane separation tank 1 and raise the solubility COD inside themembrane separation tank 1, which makes it more difficult to put themembrane separation apparatus back in operation smoothly, but with thehydrophobic porous membrane hydrophilizing agent of the presentinvention, the increase in the solubility COD inside the membraneseparation tank 1 can be suppressed. A known method can be used tomeasure COD, but an example is to measure the absorbancy as set forth inJIS K 0102. For example, when water is passed through the porousmembrane at an injection rate of 0.01 m³/m²·D per unit of surface area,it is favorable if the COD value becomes the value prior tohydrophilization treatment within 5 days, and preferably within 4 days.

(4) Method for Testing and Hydrophilizing Membrane Module (4-1) Methodfor Testing Membrane Module

As mentioned above, a membrane module usually has a main body, an inlet,an outlet, and a porous membrane, and the porous membrane is linked tothe inside of the main body so as to divide the main body into to afirst chamber having an inlet and a second chamber having an outlet.However, if there should be any defects (such as holes, cracks,incomplete linkage, or clogging of the porous membrane) in the linkedportion between the porous membrane and the main body interior, or inthe members themselves (the main body, inlet, outlet, porous membrane,etc.), this membrane module will not function as intended. Therefore, atest for these defects must be conducted.

What is called the “bubble point method” is a typical way to test aproduct. This method was originally developed for the purpose ofevaluating pore size, but because of its simplicity, it is often usedtoday in the completeness testing of microfiltration membranes andultrafiltration membranes. This method is described in JIS K 3832,“Bubble Point Test Method for Microfiltration Membrane Elements andModules.”

In specific terms, the method of the present invention for testing amembrane module comprises the steps of:

(1) immersing a membrane module in the hydrophobic porous membranehydrophilizing agent of the present invention;

(2) introducing a test gas through an inlet, passing it through thehydrophobic porous membrane, and discharging it through an outlet; and

(3) observing bubbles discharged from the membrane module.

Preferably, step (2) is conducted by:

(i) introducing the test gas through the inlet of the membrane modulewhile everything beyond the outlet is closed off;

(ii) gradually pressurizing the test gas;

(iii) using the pressurized test gas to push water out of the pores inthe hydrophobic porous membrane; and

(iv) discharging the test gas out through the hydrophobic porousmembrane.

If the membrane is damaged or has a large hole in it, air will begin topermeate at a far lower pressure than expected, and it is from this thata defect in the membrane is detected.

The hydrophobic porous membrane hydrophilizing agent in which themembrane module is immersed here contains the above-mentionedlow-frothing surfactant of the present invention, any desired solvent,and so forth. The test gas may be introduced as soon as the membranemodule is immersed, but it is preferable to leave the membrane modulesoaking for a specific length of time because the hydrophilization willbe more complete. The immersion time of the membrane module is from 30seconds to 30 minutes, for example, and preferably from 5 to 20 minutes.

Air, nitrogen, argon, or another such inert gas, or the like can be usedas the test gas. According to JIS K 3832, “Bubble Point Test Method forMicrofiltration Membrane Elements and Modules,” the test is conducted bygradually introducing the test gas up to the target pressure, rangingbetween 5 kPa and 1 MPa. If a large area is being sought, such as adefect in a potted portion, for example, it can be found at a relativelylow pressure, so the pressurization range may be about 10 to 100 kPa.

The test gas introduced into the membrane module is discharged from theoutlet, but the end of the outlet may be closed to conduct a defect testof the connected portions of the inlet, outlet, main body, and so forth.The test gas is introduced into the membrane module, and the module isvisually observed to look for bubbles released from the membrane moduleas a whole or from its inlet, outlet, or main body, bubbles releasedfrom the connected portions between the various components, and bubblesreleased from the hydrophobic porous membrane and the portion where itis connected to the main body.

If a liquid with low surface tension is used, the same defect locationdetection will be possible at a lower measured pressure value than withpure water, so this also improves incidental effects such as being ableto conduct the test in a form that leaves no pressure load hysteresis inthe membrane module.

Thus, if the defect test is conducted in the hydrophobic porous membranehydrophilizing agent of the present invention, passage of the test gaswill dry the hydrophobic porous membrane less and render it lesshydrophobic, and the decrease in permeation performance that wouldaccompany higher hydrophobicity can be suppressed. This is because whenthe test gas is passed through the membrane, contact with thehydrophobic porous membrane hydrophilizing agent of the presentinvention results in the hydrophobic porous membrane being spontaneouslywetted with water. Also, because the low-frothing surfactant of thepresent invention is used, any bubbles produced from defect portionswill collect on the water surface, so there will be no difficulty inlocating the defects. In other words, when a low-frothing surfactant isused, even if a test gas is introduced from inside the membrane module,there will be no foaming of the solution in which the membrane module isimmersed, and even if there should be a tiny amount of foaming, thebubbles will burst right away, so the test can be carried outcontinuously.

Furthermore, this configuration avoids the problems such as solventstorage encountered when a hydrophilizing agent such asglycerol-polyethylene glycol-alcohol.

(4-2) Method for Testing and Hydrophilizing Membrane Module

After the bubbles are observed and the defect test is conducted asabove, a step (4) of drying the membrane module may be furtherconducted, and the membrane module subjected to a hydrophilizationtreatment. This hydrophilization treatment is mainly conducted byimmersing the membrane module from the above-mentioned step (1) in thehydrophobic porous membrane hydrophilizing agent of the presentinvention, but if drying is performed in step (4) after this, theproduct can be distributed in a dry state in which the surface of thehydrophobic porous membrane has been hydrophilized, the treatment liquidcan be passed through at a high level of permeation performance withoutfurther performing a hydrophilization treatment at the time of use, itwill be possible to provide a membrane module product with which theinitial flow of treatment liquid recovered as waste is kept to aminimum. Also, since the hydrophobic porous membrane hydrophilizingagent of the present invention has a low-frothing property, theabove-mentioned defect test and the hydrophilization treatment can becarried out simultaneously.

The drying temperature after the defect test is, for example, from 20 to120° C., and preferably 30 to 60° C. Sufficiently high permeationperformance can be imparted as long as the drying temperature is atleast 20° C., and the heat shrinkage of the hydrophobic porous membraneand a decrease in permeation performance due to thermal degradation ofthe hydrophobic porous membrane hydrophilizing agent of the presentinvention can be suppressed as long as the temperature is 120° C. orlower.

WORKING EXAMPLES Working Example 1

A 0.3 wt % aqueous solution of an acetylene glycol-based surfactant(Olfine EXP4036, made by Nissin Chemical Industry) (this aqueoussolution had a static surface tension of 25.8 mN/m, or 27.1 mN/m whenconverted to the static surface tension for a 0.1 wt % aqueous solution)was used as the hydrophobic porous membrane hydrophilizing agent of thepresent invention.

Comparative Example 1

A 40 wt % glycerol aqueous solution (containing 15 wt % EtOH) was usedas a hydrophilizing agent.

Comparative Example 2

A 1.0 wt % aqueous solution of Emulgen LS-106.1, a higher alcohol-basedether-type nonionic surfactant (made by Kao; surface tension of 29.5mN/m) was used as a hydrophilizing agent.

Comparative Example 3

A 30% aqueous solution of ethanol (Wako Pure Chemicals, extra purereagent, 99.5%) was used as a hydrophilizing agent.

Porous Membrane 1

A membrane composed of a hollow fiber membrane (made by MitsubishiRayon) made of a vinylidene fluoride resin and having an outsidediameter of 2.4 mm, a pore size of 0.4 μm, and a pure water permeationcoefficient of 100 m³/m²/hr/MPa was prepared as a hydrophobic porousmembrane.

Porous Membrane 2

A membrane composed of a hollow fiber membrane (made by MitsubishiRayon) made of a polyethylene resin and having an outside diameter of0.54 mm, a pore size of 0.4 μm, and a pure water permeation coefficientof 30 m³/m²/hr/MPa was prepared as a hydrophobic porous membrane.

Membrane Module

A plurality of the above-mentioned porous membranes 1 were bundled toproduce a membrane module having a membrane surface area of 4.4 m².

(1) Frothing Test

The hydrophobic porous membrane hydrophilizing agent of Working Example1 was subjected to a frothing test by the Ross-Miles method. This testwas conducted as set forth in JIS K 3362. Water was added to the aqueoussolution of the hydrophobic porous membrane hydrophilizing agent inWorking Example 1 to produce a 0.1 wt % aqueous solution. After this,the bubble height was measured at 25° C., both immediately afterfrothing and 5 minutes after frothing.

Also, the surfactant of Comparative Example 2 was subjected to the samefrothing test by the Ross-Miles method.

The results are given in Table 1 below.

TABLE 1 Bubble height Bubble height 5 immediately after minutes afterfrothing frothing Working Example 1 27 mm ≦10 mm Compar. Example 2 87 mm  48 mm

(2) Defect Test Test 1

Porous membrane 1 was immersed for 10 minutes in the hydrophobic porousmembrane hydrophilizing agent of Working Example 1, after whichpressurized air (50 kPa) was introduced through the inlet of themembrane module, and the outlet and inlet were blocked off and sealed.Frothing occurred from defect portions of the membrane module, butbecause the bubbles thus produced burst right away, this did not hinderfinding the defect points, and continuing the defect test was easy.

Test 2

A membrane module was tested in the same manner as in Test 1, exceptthat porous membrane 2 was used instead of porous membrane 1. Frothingoccurred from defect portions of the membrane module, but because thebubbles thus produced burst right away, this did not hinder finding thedefect points, and continuing the defect test was easy.

Comparative Test 1

A membrane module was tested in the same manner as in Test 1, exceptthat it was immersed for 30 minutes in the surfactant of ComparativeExample 2 instead of being immersed for 10 minutes in the hydrophobicporous membrane hydrophilizing agent of Working Example 1. The bubblesproduced from defect portions of the membrane module foamed on the watersurface, and these bubbles tended to remain without bursting on thewater surface. This made the defect test difficult.

(3) Hydrophilicity and COD Test Test 1

A porous membrane 1 that was free of defects was immersed for 10 minutesin the hydrophobic porous membrane hydrophilizing agent of WorkingExample 1 and subjected to a hydrophilization treatment. After this, theporous membrane 1 was dried for 4 hours at 50° C., and its pure waterpermeation coefficient in water was measured. The pure water permeationcoefficient was calculated from the following equation.

Pure water permeation coefficient={amount of pure water permeation(m³)}/{surface area of porous membrane (m²)}/{permeation time(hr)}/{pressure of pure water (MPa)}

Also, to measure the amount of elution of the hydrophobic porousmembrane hydrophilizing agent from the hydrophobic porous membrane,water was passed through the membrane module at a pressure of 0.1 MPa,and the COD_(Mn) value in the membrane filtration water 30 minutes afterthe start of water passage was found. COD_(Mn) was measured in thepresent invention by using an absorbancy type of COD_(Mn) measurementset (made by Central Kagaku) according to JIS method (JIS K 0102).

Test 2

A porous membrane 2 that was free of defects was immersed for 10 minutesin the hydrophobic porous membrane hydrophilizing agent of WorkingExample 1 and subjected to a hydrophilization treatment. After this, theporous membrane 2 was dried for 4 hours at 50° C. and its pure waterpermeation coefficient in water was measured in the same manner as inTest 1.

Comparative Test 1

A porous membrane 1 that was free of defects and that had not undergonehydrophilization treatment was dried for 4 hours at 50° C. After this,pure water permeation coefficient in water was measured.

Comparative Test 2

A porous membrane 1 that was free of defects was immersed for 10 secondsthe 40 wt % glycerol aqueous solution of Comparative Example 1 andsubjected to a hydrophilization treatment. After this, the porousmembrane 1 was dried for 4 hours at 50° C. and its pure water permeationcoefficient in water was measured in the same manner as in Test 1. Waterwas passed through the membrane module at a pressure of 0.1 MPa, and theCOD_(Mn) value 30 minutes after the start of water passage was measured.

Comparative Test 3

A porous membrane 1 that was free of defects was immersed for 10 minutesin the surfactant aqueous solution of Comparative Example 2 andsubjected to a hydrophilization treatment. After this, the porousmembrane 1 was dried for 4 hours at 50° C. and its pure water permeationcoefficient in water was measured in the same manner as in Test 1.

The results are given in Table 2 below.

TABLE 2 Pure water permeation coefficient (m³/m²/hr/MPa) COD_(Mn) PorousHydrophilizing Before After value membrane agent drying drying(mg/liter) Test 1 porous Working 100 96 ≦10 membrane 1 Example 1 Test 2porous Working 30 28 ≦10 membrane 2 Example 1 Comp. porous none 100 5 0Test 1 membrane 1 Comp. porous Comp. 100 93 2200 Test 2 membrane 1Example 1 Comp. porous Comp. 100 85 50 Test 3 membrane 1 Example 2

Test 1 and Comparative Test 2 indicated about the same pure waterpermeation coefficient as when hydrophilization and water replacementwere performed with ethanol (30% aqueous solution). However, the amountof glycerol elution was larger in Comparative Test 2.

(4) Hydrophilization Test of Hydrophobic Porous Membrane in MembraneSeparation Apparatus Test 3

The membrane separation tank 1 of a membrane separation apparatus havinga 0.7 m³ membrane separation tank 1 was filled with ordinary householdwastewater, before and after treatment such as condensation settling, asthe treatment liquid 3. Active sludge was added as microbes to themembrane separation tank 1.

The membrane module 2 was immersed and installed in the membraneseparation tank 1, and the hydrophobic porous membrane hydrophilizingagent of Working Example 1 was injected from the pipe 5 into themembrane module 2 in an amount of 2 liters per square meter of membranesurface area and at an injection rate of 0.01 m³/m²·D per unit ofsurface area. Upon completion of the injection, the membrane module wasallowed to stand for 60 minutes and hydrophilization treatment wasperformed. Water that had been filtered through a hollow fiber membrane(pore size of 0.1 μm) was injected from the pipe 5 into the membranemodule 2 in an amount of 2 liters per square meter of membrane surfacearea and at an injection rate of 0.01 m³/m²·D per unit of surface areato push the hydrophobic porous membrane hydrophilizing agent of WorkingExample 1 into the membrane separation tank 1. After thehydrophilization treatment, air was immediately introduced through thediffuser pipe 4 to discharge the treatment liquid from the pipe 5, andthe operation of the membrane separation apparatus was commenced. Oneweek later the pure water permeation coefficient of the hydrophobicporous membrane and the COD_(Mn) value of the treated liquid weremeasured.

These results are given in Table 3 below.

TABLE 3 Pure water permeation coefficient COD_(Mn) value Test 3(m³/m²/hr/MPa) (mg/liter) before 100 12.0 hydrophilization treatmentafter 0 minutes 98 89.3 after 1 hour 100 16.3 after 24 hours 99 13.6after 2 days 100 13.5 after 3 days 97 12.3 after 4 days 100 11.8 after 5days 100 11.8 after 6 days 100 12.0 after 7 days 97 11.3

It was confirmed that over the course of 1 week after thehydrophilization treatment, there was no sudden decrease in the purewater permeation coefficient, and the hydrophilization treatment wasbeing sufficiently carried out. Also, it was possible for smoothoperation to continue from immediately after the hydrophilizationtreatment, without the treatment liquid 3 foaming, or the bubbles andtreatment liquid 3 overflowing from the membrane separation tank 1 dueto aeration from the diffuser pipe 4.

The COD_(Mn) value of the treated liquid was seen to rise about 4mg/liter in the initial flow after 15 minutes, as compared to before thehydrophilization treatment, but 24 hours after operation had commenced,the COD_(Mn) value had decreased to the same level as that prior to thehydrophilization treatment.

Comparative Test 4

The pure water permeation coefficient of the hydrophobic porous membraneand the COD_(Mn) value of the treated liquid were measured in the samemanner as in Test 3, except that the 40 wt % glycerol aqueous solutionof Comparative Example 1 was injected from the pipe 5 into the membranemodule 2.

These results are given in Table 4 below.

TABLE 4 Pure water permeation coefficient COD_(Mn) value ComparativeTest 4 (m³/m²/hr/MPa) (mg/liter) before 100 11 hydrophilizationtreatment after 0 minutes 98 15,700 after 1 hour 100 2,200 after 24hours 99 450 after 2 days 100 120 after 3 days 97 70 after 4 days 100 35after 5 days 100 18 after 6 days 100 12.5 after 7 days 97 11.0

The COD_(Mn) value of the treated liquid was seen to rise about 2189mg/liter in the initial flow after 1 hour, as compared to before thehydrophilization treatment. The treated liquid at the start ofoperation, including the initial flow, had to be considered waste. Even24 hours after the start of operation, the concentration was higher thanbefore the hydrophilization treatment, and after 5 days the COD_(Mn)value decreased to the level prior to addition.

Comparative Test 5

The pure water permeation coefficient of the hydrophobic porous membraneand the COD_(Mn) value of the treated liquid were measured in the samemanner as in Test 3, except that the ethanol aqueous solution ofComparative Example 3 was injected from the pipe 5 into the membranemodule 2.

These results are given in Table 5 below.

TABLE 5 Pure water permeation coefficient COD_(Mn) value ComparativeTest 5 (m³/m²/hr/MPa) (mg/liter) before 100 12.0 hydrophilizationtreatment after 0 minutes 98 1942.0 after 1 hour 100 105.8 after 24hours 99 20.5 after 2 days 99 14.5 after 3 days 97 13.8 after 4 days96.5 12.0 after 5 days 94.8 11.8 after 6 days 95.3 12.0 after 7 days94.8 12.3

It was confirmed that over the course of 1 week after thehydrophilization treatment, there was no sudden decrease in the purewater permeation coefficient, and the hydrophilization treatment wasbeing sufficiently carried out. Also, although there was slight foamingof the treatment liquid 3, it was possible for smooth operation tocontinue from immediately after the hydrophilization treatment, withoutbubbles and treatment liquid 3 overflowing from the membrane separationtank 1 due to aeration from the diffuser pipe 4.

The COD_(Mn) value of the treated liquid was seen to rise about 1830mg/liter in the initial flow after 0 minutes, as compared to before thehydrophilization treatment, so the treated liquid had to be consideredwaste. It took about 1 day for the ethanol to be biodegraded, and theapparatus could not be started up quickly after the hydrophilizationtreatment.

1. A hydrophilizing agent for a hydrophobic porous membrane, wherein theagent contains a surfactant, and the surfactant has a frothability suchthat the bubble height immediately after frothing, as measured accordingto the Ross-Miles method (JIS K 3362), using a 0.1 wt % aqueous solutionof the surfactant at 25° C., is 40 mm or less.
 2. The hydrophilizingagent for a hydrophobic porous membrane according to claim 1, whereinthe surfactant has the frothability such that the bubble heightimmediately after frothing is 30 mm or less.
 3. The hydrophilizing agentfor a hydrophobic porous membrane according to claim 1, wherein thesurfactant has a frothability such that the bubble height five minutesafter frothing, as measured according to the Ross-Miles method (JIS K3362), using a 0.1 wt % aqueous solution of the surfactant at 25° C., is20 mm or less.
 4. The hydrophilizing agent for a hydrophobic porousmembrane according to claim 3, wherein the surfactant has thefrothability such that the bubble height five minutes after frothing is15 mm or less.
 5. The hydrophilizing agent for a hydrophobic porousmembrane according to claim 1, wherein the surfactant has a staticsurface tension of 30 mN/m or less when a 0.1 wt % aqueous solution ofthe surfactant is used.
 6. The hydrophilizing agent for a hydrophobicporous membrane according to claim 5, wherein the surfactant has thestatic surface tension of 29 mN/m or less.
 7. The hydrophilizing agentfor a hydrophobic porous membrane according to claim 1, wherein thesurfactant is a nonionic surfactant.
 8. The hydrophilizing agent for ahydrophobic porous membrane according to claim 1, wherein the surfactantis acetylene glycol, ethoxylated acetylene glycol, or a mixture ofthese.
 9. The hydrophilizing agent for a hydrophobic porous membraneaccording to claim 1, wherein the surfactant is selected from the groupof 2,4,7,9-tetramethyl-5-decyn-4,7-diol, 3,6-dimethyl-4-octyn-3,6-diol,3,5-dimethyl-1-hexyn-3-ol, 2,5,8,11-tetramethyl-6-dodecyn-5,8-diol, andethoxylates of these.
 10. The hydrophilizing agent for a hydrophobicporous membrane according to claim 1, wherein the surfactant iscontained in an amount of 0.05 to 5 wt % with respect to the entirehydrophobic porous membrane hydrophilizing agent.
 11. The hydrophilizingagent for a hydrophobic porous membrane according to claim 1, whereinthe agent further contains a solvent.
 12. The hydrophilizing agent for ahydrophobic porous membrane according to claim 11, wherein the solventis water.
 13. The hydrophilizing agent for a hydrophobic porous membraneaccording to claim 11, wherein the hydrophobic porous membrane is formedfrom polyvinylidene fluoride.
 14. A method for hydrophilizing ahydrophobic porous membrane, comprising a step of bringing a hydrophobicporous membrane into contact with the hydrophilizing agent for ahydrophobic porous membrane according to claim
 1. 15. The method forhydrophilizing a hydrophobic porous membrane according to claim 14,further comprising a step of drying the hydrophobic porous membrane thathas come into contact with the hydrophilizing agent for a hydrophobicporous membrane according to claim
 1. 16. The method for hydrophilizinga hydrophobic porous membrane according to claim 14, wherein theadhesion proportion of the hydrophilizing agent is 0.01 to 1.0 wt %. 17.A method for testing a membrane module having a main body, an inlet andoutlet provided to the main body, and a hydrophobic porous membraneprovided inside the main body, the method comprising the steps of: (1)immersing the membrane module in the hydrophilizing agent for ahydrophobic porous membrane according to claim 1; (2) introducing a testgas through the inlet, allowing the gas to pass through the hydrophobicporous membrane, and discharging the gas through the outlet; and (3)observing bubbles discharged from the membrane module.
 18. The methodfor testing a membrane module according to claim 17, wherein the step(2) is conducted by: (i) introducing the test gas through the inlet ofthe membrane module while everything beyond the outlet is closed off;(ii) gradually pressurizing the test gas; (iii) using the pressurizedtest gas to push water out of the pores in the hydrophobic porousmembrane; and (iv) discharging the test gas out through the hydrophobicporous membrane.
 19. A method for testing and hydrophilizing a membranemodule having a main body, an inlet and outlet provided to the mainbody, and a hydrophobic porous membrane provided inside the main body,the method comprising the steps of: (1) immersing the membrane module inthe hydrophilizing agent for a hydrophobic porous membrane according toclaim 1; (2) introducing a test gas through the inlet, allowing the gasto pass through the hydrophobic porous membrane, and discharging the gasthrough the outlet; (3) observing bubbles discharged from the membranemodule; and (4) drying the membrane module.
 20. The method for testingand hydrophilizing a membrane module according to claim 19, wherein thestep (2) is conducted by: (i) introducing the test gas through the inletof the membrane module while everything beyond the outlet is closed off;(ii) gradually pressurizing the test gas; (iii) using the pressurizedtest gas to push water out of the pores in the hydrophobic porousmembrane; and (iv) discharging the test gas out through the hydrophobicporous membrane.